Wireless devices such as cellular telephones are designed to emit electromagnetic radiation during use. Repetitive use of these devices, especially in close proximity to the human body, has been postulated to impart relatively high levels of cumulative radiation. High levels of exposure have been shown to pose a potential health risk and an increased risk of certain types of cancers in humans. Increased cancer risk is of a particular concern, considering the use of cellular telephones typically occurs close to the head and brain.
When electromagnetic waves are absorbed by an object, the energy of the waves is converted to heat. Electromagnetic waves can also be reflected or scattered, in which case their energy is redirected or redistributed. The quantity of radiant energy absorbed transmitted may be calculated by integrating radiant flux (or power) with respect to time.
Instantaneous electrical power P is given byP(t)=I(t)·V(t)  [1]where                P(t) is the instantaneous power, measured in watts (joules per second)        V(t) is the potential difference (or voltage drop) across the component, measured in volts        I(t) is the current through it, measured in amperes.        
In the case of a periodic signal s(t) of period T, like a train of identical pulses, the instantaneous power p(t)=|s(t)|2 is also a periodic function of period T. The peak power is defined by:P0=max[p(t)].  [2]
The peak power is not always readily measurable, therefore, and the average power is more commonly used as a measure of delivered power. If energy per pulse is defined as:∈pulse=∫0Tp(t)dt  [3]then the average power is defined as:
                              P          avg                =                                            1              T                        ⁢                                          ∫                0                T                            ⁢                                                p                  ⁡                                      (                    t                    )                                                  ⁢                d                ⁢                                                                  ⁢                t                                              =                                                    ϵ                pulse                            T                        .                                              [        4        ]            
A notable fraction of the power from the electromagnetic radiation emitted by a cellular telephone when in use is absorbed by the human head. The electromagnetic radiation emitted by a GSM handset, for example, has a peak power of about 2 watts. Other digital mobile technologies, such as CDMA2000 and D-AMPS, have a peak power of about 1 watt.
The specific absorption rate (“SAR”) is the rate at which energy is absorbed by the body when exposed to a radio frequency electromagnetic field. The SAR level is defined as the power of the electromagnetic radiation absorbed per mass of tissue in units of watts per kilogram (W/kg) and is averaged over a small sample volume. SAR maximum levels for cellular telephones have been set by governmental regulating agencies in many countries. In the United States, the Federal Communications Commission (FCC) has set a SAR limit of 1.6 W/kg, averaged over a volume of 1 gram of tissue, for the head. In Europe, the limit is 2 W/kg, averaged over a volume of 10 grams of tissue.
One well-understood effect of electromagnetic radiation is dielectric heating, in which any dielectric material (such as living tissue) is heated by rotations of polar molecules induced by the electromagnetic field. In the case of a person using a cellular telephone, most of the heating effect will occur at the surface of the head, causing its temperature to increase by a fraction of a degree. In this case, the level of temperature increase is an order of magnitude less than that obtained during the exposure of the head to direct sunlight. The brain's blood circulation is capable of disposing of excess heat by increasing local blood flow. However, other areas of the body, such as the cornea of the eye, do not have this temperature regulation mechanism. Exposure of 2-3 hours duration has been reported to produce cataracts in rabbits' eyes at SAR values from 100-140 W/kg, which produced lenticular temperatures of 41° C.
Other “non-thermal” effects are less well understood. For example, thermoreceptor molecules in cells activate a variety of secondary and tertiary messenger systems, in order to defend the cell against metabolic cell stress caused by heat. The increases in temperature that cause these changes are too small to be detected by current studies. Further, the communications protocols used by mobile phones often result in low-frequency pulsing of the carrier signal. Whether these modulations have biological significance has been subject to debate.
A study published in 2011 by The Journal of the American Medical Association conducted using fluorodeoxyglucose injections and positron emission tomography concluded that exposure to radiofrequency signal waves within parts of the brain closest to the cellular telephone antenna resulted in increased levels of glucose metabolism, but the clinical significance of this finding is unknown.
Despite differing opinions among researchers, evidence has accumulated that supports the existence of complex biological effects of weaker non-thermal electromagnetic fields, and modulated RF and microwave fields. The World Health Organization has classified radiofrequency electromagnetic radiation as a possible group 2b carcinogen. This group contains possible carcinogens with weaker evidence, at the same level as coffee and automobile exhaust.
At frequencies higher than radio frequencies (e.g., ultraviolet light), the biological effects of radiation are more pronounced. Radiation at these frequencies has sufficient energy (directly or indirectly) to damage biological molecules through ionization. All frequencies of UV radiation have been classed as Group I carcinogens by the World Health Organization. Ultraviolet radiation from sun exposure is the primary cause of skin cancer.
Thus, at UV frequencies and higher, electromagnetic radiation becomes ionizing and so does far more damage to biological systems than simple heating. “Ionization” produces ions and free radicals in materials (including living tissue) with very little heating, resulting in severe damage with little or no warning. Radiation in this frequency range is currently considered far more dangerous than the rest of the electromagnetic spectrum. But, it is postulated that low frequencies, perhaps as low as radio frequencies, can produce ionization effects, like those of X-rays, but at statistically less significant numbers. Over time, the cumulative effects of radio frequency radiation on living tissue may be significant enough to cause tissue damage.
Radiation exposure may be reduced by decreasing the duration of exposure or increasing the distance between the source of the radiation and the subject. Alternatively, increasing shielding between the radiation source and the subject will also reduce radiation exposure.
The prior art has attempted to provide electromagnetic shielding solutions for use with cellular telephones but has not been completely successful.
For example, U.S. Pat. No. 7,242,507 to Yen discloses an electromagnetic wave absorptive film. The film is comprised of a compound layer and a reflective layer. However, the film in Yen requires the embedding of absorbing grains into the compound layer leading to a complex manufacturing process. Further, the film cannot be used on cellular telephones having touch-sensitive screens.
U.S. Publication No. 2004/0198264 to Saur, et al. discloses a shielding that includes a flexible conductive sheet and an adhesive for attachment to a housing of a wireless telephone. However, the shielding apparatus disclosed in Saur cannot be used with cellular telephones having touch-sensitive screens.
PCT Publication No. WO 2010/115159 to Bradshaw, et al. discloses metal nanopowders for use as radiation shields. However, to be effective the nanoparticles and nanopowders in Bradshaw require two layers, a core and an outer layer. Further, the outer layer requires a group of several organic substituents, which require a complicated and labor intensive manufacturing process.
The prior art fails to disclose or suggest a radiation shield for a handheld cellular telephone having a simple construction and a wide range of uses including uses with touch-sensitive screens. Therefore, there is a need in the art for a radiation shield for cellular telephones such as cellular telephones that is easy to manufacture and adaptable for use on a wide range of cellular telephones, including devices with touch-sensitive screens.